Edge Hydrogenation Research Articles

Spin filter effects in zigzag-edge graphene nanoribbons with symmetric and asymmetric edge hydrogenations

Using the non-equilibrium Green’s function method combined with the density functional theory, we investigate the magnetism and spin-dependent transport properties of symmetric and asymmetric zigzag–edged graphene nanoribbons (ZGNRs) passivated by monohydrogenation with [1, −1] magnetism configuration. The symmetric model H–6ZGNR–H shows a dual spin filter effect, but asymmetric H–5ZGNR–H behaves as a conductor with linear current–voltage relationships. We also investigate the magnetism and spin-dependent transport properties of H2–ZGNR–H with asymmetric edge hydrogenations, the perfect dual spin filtering effect is observed in both H2–5ZGNR–H and H2–6ZGNR–H. The transmission pathways and PDOS demonstrate that the edge of C-H2 bonds have important effects on the magnetism and spin-dependent transport properties as compared with the edge of C–H bonds.

Rectifying performance in zigzag graphene nanoribbon heterojunctions with different edge hydrogenations

Abstract Using nonequilibrium Greenʼs functions in combination with the density functional theory, we investigated the electronic transport behaviors of zigzag graphene nanoribbon (ZGNR) heterojunctions with different edge hydrogenations. The results show that electronic transport properties of ZGNR heterojunctions can be modulated by hydrogenations, and prominent rectification effects can be observed. We propose that the edge dihydrogenation leads to a blocking of electronic transfer, as well as the changes of the distribution of the frontier orbital at negative/positive bias might be responsible for the rectification effects. These results may be helpful for designing practical devices based on graphene nanoribbons.

Tuning the electronic and magnetic properties of zigzag silicene nanoribbons by edge hydrogenation and doping

We have performed density-functional-theory calculations to study the electronic and magnetic properties of edge-hydrogenated and edge-doped zigzag silicene nanoribbons (ZSiNRs). The formation energies of monohydrogenated and dihydrogenated edges slightly depend on the width of ZSiNRs, whereas the electronic and magnetic properties of ZSiNRs can be tuned by the different forms of edge hydrogenation. The dihydrogenated edges can effectively stabilize the antiferromagnetic semiconducting ground state of ZSiNRs. Moreover, nitrogen- or phosphorus-doped monohydrogenated ZSiNR is a magnetic semiconductor, and becomes a half-metal under a transverse electric field. Owing to the compelling electronic and magnetic properties, silicene nanoribbons hold great promise for application in nanodevices.

Effects of edge hydrogenation in zigzag silicon carbide nanoribbons: stability, electronic and magnetic properties, as well as spin transport property

Based on density functional theory and nonequilibrium Green's function method, we systematically investigated the hydrogenated effects on the stability, electronic and magnetic properties, as well as electronic spin transport property of an N chains zigzag silicon carbide nanoribbon (N-ZSiC NR). Our calculated results indicate that by controlling the hydrogen content of the environment, one can get three types of stable edge hydrogenated ZSiC NRs. They are: (a) each edge Si and C atom bonded with one hydrogen atom (N-ZSiC-1H1H), (b) each edge Si atom bonded with two H atoms and each edge C bonded with one H atom (N-ZSiC-2H1H), and (c) each edge Si and C atom bonded with two H atoms (N-ZSiC-2H2H). It was unexpectedly found that N-ZSiC-1H1H NR, which has been studied theoretically to a large extent, is stable only at extremely low ultravacuum pressures. Under more standard conditions, the most stable edge hydrogenated structure is N-SiC-2H2H NR. More interestingly, when N ≤ 4, the N-ZSiC-2H2H NR is a nonmagnetic semiconductor, while when 5 ≤ N ≤ 7, it is a ferrimagnetic ferromagnetic semiconductor. When N ≥ 8, the N-SiC-2H2H NR turns into a ferrimagnetic half-metallic. As regards the N-ZSiC-2H1H NR, when N ≤ 12, it is a ferromagnetic semiconductor, while when N ≥ 13, it becomes a ferromagnetic half-metallic. These results manifest that by controlling the hydrogen content of the environment and the temperature, as well as the ribbon width N, one can precisely modulate the electronic and magnetic properties of N-ZSiC NRs, which endows ZSiC NRs with many potential applications in spintronics and nanodevices.

Excellent catalytic effects of highly crumpled graphenenanosheets on hydrogenation/dehydrogenation of magnesium hydride

Highly crumpled graphene nanosheets (GNS) with a BET surface area as high as 1159 m(2) g(-1) was fabricated by a thermal exfoliation method. A systematic investigation was performed on the hydrogen sorption properties of MgH(2)-5 wt% GNS nanocomposites acquired by ball-milling. It was found that the as-synthesized GNS exhibited a superior catalytic effect on hydrogenation/dehydrogenation of MgH(2). Differential Scanning Calorimetry (DSC) and isothermal hydrogenation/dehydrogenation measurements indicated that both hydrogen sorption capacity and dehydrogenation/hydrogenation kinetics of the composites improved with increasing milling time. The composites MgH(2)-GNS milled for 20 h can absorb 6.6 wt% H(2) within 1 min at 300 °C and 6.3 wt% within 40 min at 200 °C, even at 150 °C, it can also absorb 6.0 wt% H(2) within 180 min. It was also demonstrated that MgH(2)-GNS-20 h could release 6.1 wt% H(2) at 300 °C within 40 min. In addition, microstructure measurements based on XRD, SEM, TEM as well as Raman spectra revealed that the grain size of thus-prepared MgH(2)-GNS nanocomposites decreased with increasing milling time, moreover, the graphene layers were broken into smaller graphene nanosheets in a disordered and irregular manner during milling. It was confirmed that these smaller graphene nanosheets on the composite surface, providing more edge sites and hydrogen diffusion channels, prevented the nanograins from sintering and agglomerating, thus, leading to promotion of the hydrogenation/dehydrogenation kinetics of MgH(2).

Birch Reduction of Graphite. Edge and Interior Functionalization by Hydrogen

The Birch reduction (lithium in liquid ammonia) of graphite gives a highly reduced, exfoliated product that is free of lithium. Edge and interior hydrogenation were demonstrated by solid-state (13)C NMR spectroscopy. Elemental analysis of a carefully purified sample allows the chemical composition to be expressed as (C(1.3)H)(n). Atomic force microscopy images showed that the reduced graphene was highly exfoliated. Hydrogen mapping by electron energy loss spectroscopy showed that the entire surface of the reduced sample was covered by hydrogen, consistent with the NMR studies also indicating that hydrogen was added in interior positions of the graphene lattice as well as along the edge. A large band gap (4 eV) further establishes the high level of hydrogenation.

Orbital symmetry induced conductance switching in a graphene nanoribbon heterojunction with different edge hydrogenations

First principles calculations are performed to investigate the electron transport through a zigzag-edged graphene nanoribbon (ZGNR) heterojunction constructed by connecting a monohydrogenated ZGNR and a dihydrogenated ZGNR and its response to external magnetic fields. It is found that the heterojunction can be switched between a conducting state and an insulating state by tuning the magnetic fields. It arises from the matching or mismatching between the π or π* states of the two ribbons under different magnetic fields. This mechanism of conductance switching by tuning the orbital symmetry can be considered in the future design of graphene based electronic devices.

Band gap engineering in armchair-edged graphene nanoribbons by edge dihydrogenation

We report our first principles study of dihydrogenation effects on the electronic structures of armchair-edged graphene nanoribbons (AGNRs). It is found that dihydrogenation brings completely different effects from mono-hydrogenation. For AGNRs, when the edge hydrogenation scheme varies from H:H to H:H2 and then to H2:H2, the band gap of the ribbon will change in a “V” style, “⧹” style or “Λ” style, depending on whether the ribbon width n=3p or 3p+1 or 3p+2. Further analysis shows that this interesting change arises from the decrease of the effective ribbon width induced by dihydrogenation. In addition, the band structures of H2:H2 (n−2)-AGNRs, H:H2 (n−1)-AGNRs and H:H n-AGNRs around the Fermi level are very similar and their gaps are slightly different. Like in the band gap, the family behaviors are also observed in the formation energy. The trend in band gap change is consistent with the trend in the formation energy in reflecting the relative stability of the AGNRs with different hydrogenations. These findings provide a basis for band gap engineering with different edge hydrogenations in AGNRs and may find applications in the design of graphene based devices.

Electronic Transport of Zigzag Graphene Nanoribbons with Edge Hydrogenation and Oxidation

By using non-equilibrium Green's functions in combination with the density-functional theory, we study the effect of zigzag graphene nanoribbons with edge hydrogenation and oxidation on transport properties. We find that for the ferromagnetic (FM) configuration the ZGNRs with CH2-CH group exhibit spin diode effect in which only one spin can occur under positive bias while the other spin occurs under negative bias. In the antiferromagnetic (AF) state the symmetric ZGNRs with CH2-CH group show the spin filter effect within some specific energy windows. However, the asymmetric ZGNRs with CH2 -CH group do not show such a spin filter effect. We also find that the symmetric and asymmetric ZGNRs with C2O-CH group in AF configurations show similar transport behaviors at the Fermi level. Such ZGNRs might be exploited in spintronic nanodevices.

Honeycomb-Patterned Quantum Dots beyond Graphene

Graphene quantum dots (QDs) hold great promises in spintronics. Here, we report our predictions of honeycomb-patterned QDs beyond graphene, on the basis of first-principles calculations and an extended Hubbard model. Our calculations showed that the electronic structures and spin-polarization of boron nitride (BN) and silicon carbide (SiC) QDs can be well tuned by controlling the shape and size of the QDs. Edge hydrogenation can not only greatly improve the stability but also diminish the spin-polarization of BN-QDs. Triangular SiC-QDs have spin-polarized ground states, and the magnetic moments increase with the increase of QD size. Hexagonal SiC-QDs, however, possess spin-unpolarized ground states whose energy gaps decrease with the increase of QD size. To understand the origins of the composition- and shape-dependent spin-polarization of these honeycomb-patterned QDs, we extended the single-orbital Hubbard model of graphene QDs by taking into account the onsite energy differences of the two sublattices. Our extended Hubbard model reproduces well the results of first-principles calculations and offers a simple model to predict the electronic structures of honeycomb-patterned QDs.

Atomistic study of the competitive relationship between edge dislocation motion and hydrogen diffusion in alpha iron

Abstract

Doping induced spin filtering effect in zigzag graphene nanoribbons with asymmetric edge hydrogenation

The magnetic and spin dependent transport properties of asymmetrically hydrogenated zigzag graphene nanoribbons, which are C–H2 bonded at one edge while C–H bonded at the other, are investigated from first-principles calculations. Due to their special distributions of the density of states near Fermi level, a perfect (100%) spin filtering effect can be achieved in such graphene nanoribbons through p-type or n-type doping. Moreover, a negative differential resistance effect is observed in both doping case, which results from the reducing of conductance near Fermi level with increasing bias voltage.

Electronic and magnetic properties of superlattices of graphene/graphane nanoribbons with different edge hydrogenation

Zigzag graphene nanoribbons patterned on graphane are studied using spin-polarized ab initio calculations. We found that the electronic and magnetic properties of the graphene/graphane superlattice strongly depends on the degree of hydrogenation at the interfaces between the two materials. When both zigzag interfaces are fully hydrogenated, the superlattice behaves like a freestanding zigzag graphene nanoribbon, and the magnetic ground state is antiferromagnetic. When one of the interfaces is half hydrogenated, the magnetic ground state becomes ferromagnetic, and the system is very close to being a half metal with possible spintronics applications whereas the magnetic ground state of the superlattice with both interfaces half hydrogenated is again antiferromagnetic. In this last case, both edges of the graphane nanoribbon also contribute to the total magnetization of the system. All the spin-polarized ground states are semiconducting, independent of the degree of hydrogenation of the interfaces. The ab initio results are supplemented by a simple tight-binding analysis that captures the main qualitative features. Our ab initio results show that patterned hydrogenation of graphene is a promising way to obtain stable graphene nanoribbons with interesting technological applications.

Calculation of near K edge x-ray absorption spectra and hydrogen bond network in ice XIII under compression

The hydrogen bond network, oxygen K edge x-ray absorption spectra (XAS), and electronic structure of ice XIII under compression have been extensively studied by density functional theory (DFT). We showed that DFT methods yield a ground state consistent with previous neutron scattering experiment and a few low-enthalpy metastable states are likely to coexist from the total enthalpy calculations. Oxygen K edge XAS of four low-enthalpy configurations was studied with the aim to shed light on the local structure in these configurations. We demonstrated that pre-edge of oxygen K edge XAS is a common feature appearing in all these four structures while major spectral differences exist in the main peak area. Therefore, we arrived at the conclusion that the main peak is more sensitive to the local hydrogen bond environment and could be used as an effective tool to distinguish these four configurations. We also found that the pre-edge has main contribution from O 1s-4a1 transitions and its intensity was suppressed by pressure while the main peak is mostly coming from O 1s-2b2 transitions.

Effect of lattice topology on the adsorption of benzyl alcohol on kaolinite surfaces: Quantum chemical calculations of geometry optimization, binding energy, and NMR chemical shielding

We investigate the effect of the lattice topology (ideal hexagonal rings vs. ditrigonal rings) and cluster size of model clusters (three-ring vs. seven-ring clusters) on the nature of benzyl alcohol adsorption on kaolinite surfaces using quantum chemical calculations with an emphasis on the equilibrium configuration, binding energy, and NMR chemical shielding tensors. The optimized structure of benzyl alcohol adsorbed on the tetrahedral layer of kaolinite varies according to the type and size of model cluster. While the calculated binding energy varies with the level of theory and the basis sets used for the calculations, the binding energies between benzyl alcohol and seven-ring clusters are smaller than those between benzyl alcohol and three-ring clusters partly due to the edge hydrogen for the latter. The results also indicate a stronger binding energy between benzyl alcohol and octahedral surfaces than between benzyl alcohol and tetrahedral layers. Although the calculated binding energies for seven-member rings with varying lattice topologies are rather similar, the detailed optimized structures are distinct, demonstrating the effect of lattice topology on the nature of adsorption. The optimized structures and binding energies indicate that an intermediate degree of hydrogen bonding is dominant for the three-member silicate rings and that the interaction between the benzene ring and basal O atoms in the seven-member rings is characterized by a weak hydrogen bond and dispersion force. The calculated 17O isotropic chemical shieldings of some basal O atoms decrease up to ~4–5 ppm after the adsorption (with an estimated uncertainty of ~2 ppm). Since the high-resolution 17O 3QMAS NMR spectroscopy of layer silicates yielded a resolution of 1–2 ppm for the basal oxygen sites in the layer silicates in previous work, the NMR technique may be useful in exploring the nature of adsorption between organic molecules and silicate surfaces, whereas further computational studies on the effect of the basis sets, the surface coverage, and the types of diverse organic molecules with larger model clusters for surfaces remain to be explored.

Electronic structures of zigzag graphene nanoribbons with edge hydrogenation and oxidation

Using the ab initio density-functional theory method and local spin-density approximation, we calculated the electronic band structures of H or H2 edge-hydrogenated zigzag graphene nanoribbons (ZGNRs) as well as COH, CO, or C2O edge-oxidized ZGNRs. We found that the OH group yields almost the same band structure as the $s{p}^{2}$ hybridization of H edge, and that the ketone (CO) and ether (C2O) groups result in band structures similar to those of $s{p}^{3}$ hybridization of H2 edge. Compared to H passivation, edge oxidation by the ketone or the ether group is energetically more favorable, suggesting that the GNR's edges will be oxidized in the presence of oxidizing species. Edge oxidized GNRs show metallic band structures caused by the larger electronegativity of oxygen relative to carbon, and these findings raise a question about the physical origins of the experimental observations of semiconducting GNRs. Such discrepancy suggests that more realistic modeling of GNR edge structures will be necessary to understand the experimental findings.

Airport electric vehicle powered by fuel cell

Nowadays, new technologies and breakthroughs in the field of energy efficiency, alternative fuels and added-value electronics are leading to bigger, more sustainable and green thinking applications. Within the Automotive Industry, there is a clear declaration of commitment with the environment and natural resources. The presence of passenger vehicles of hybrid architecture, public transport powered by cleaner fuels, non-aggressive utility vehicles and an encouraging social awareness, are bringing to light a new scenario where conventional and advanced solutions will be in force. This paper presents the evolution of an airport cargo vehicle from battery-based propulsion to a hybrid power unit based on fuel cell, cutting edge batteries and hydrogen as a fuel. Some years back, IBERIA (Major Airline operating in Spain) decided to initiate the replacement of its diesel fleet for battery ones, aiming at a reduction in terms of contamination and noise in the surrounding environment. Unfortunately, due to extreme operating conditions in airports (ambient temperature, intensive use, dirtiness, …), batteries suffered a very severe degradation, which took its toll in terms of autonomy. This reduction in terms of autonomy together with the long battery recharge time made the intensive use of this fleet impractical in everyday demanding conditions.

Role of Hoogsteen Edge Hydrogen Bonding at Template Purines in Nucleotide Incorporation by Human DNA Polymerase ι

Human DNA polymerase iota (Pol iota) differs from other DNA polymerases in that it exhibits a marked template specificity, being more efficient and accurate opposite template purines than opposite pyrimidines. The crystal structures of Pol iota with template A and incoming dTTP and with template G and incoming dCTP have revealed that in the Pol iota active site, the templating purine adopts a syn conformation and forms a Hoogsteen base pair with the incoming pyrimidine which remains in the anti conformation. By using 2-aminopurine and purine as the templating residues, which retain the normal N7 position but lack the N(6) of an A or the O(6) of a G, here we provide evidence that whereas hydrogen bonding at N(6) is dispensable for the proficient incorporation of a T opposite template A, hydrogen bonding at O(6) is a prerequisite for C incorporation opposite template G. To further analyze the contributions of O(6) and N7 hydrogen bonding to DNA synthesis by Pol iota, we have examined its proficiency for replicating through the (6)O-methyl guanine and 8-oxoguanine lesions, which affect the O(6) and N7 positions of template G, respectively. We conclude from these studies that for proficient T incorporation opposite template A, only the N7 hydrogen bonding is required, but for proficient C incorporation opposite template G, hydrogen bonding at both the N7 and O(6) is an imperative. The dispensability of N(6) hydrogen bonding for proficient T incorporation opposite template A has important biological implications, as that would endow Pol iota with the ability to replicate through lesions which impair the Watson-Crick hydrogen bonding potential at both the N1 and N(6) positions of templating A.

Normal Coordinate Analysis and Vibrational Spectra of Aromatic Carbon Nanostructures

Theoretical infrared absorption spectra of aromatic ring molecules having up to 102 carbon atoms and with various edge hydrogenations have been obtained using a classic mechanical model and a simplified valence force field. Force constants have been adapted from those available for smaller molecules. Spectral line intensities are calculated in a double harmonic approximation with effective atomic charges obtained using a Huckel tight-binding Hamiltonian. Vibrational modes of clusters of like aromatic ring molecules are obtained through introduction of an interlayer force constant. These modes are predicted to occur in the wavelength range between 80 and 400 μm. We explore the effect of dehydrogenation on these spectra as well as hydrogenation of terminal C atoms in either aromatic or aliphatic form. Many spectra exhibit a quasi-continuum between 6 and 9 μm that arises from the overlap of many vibrational modes in this region. With CH2 edge groups, we find a new spectral feature near 16 μm and the aliphatic CH2 symmetric stretch vibration is found to shift from 3.5 to 3.3 μm when these groups are present at terminal sites on aromatic carbon skeletons. The relevance of calculated spectra to those of astronomical sources is briefly discussed.

Environment of tryptophan side chains in proteins

Although relatively rare, the tryptophan residue (Trp), with its large hydrophobic surface, has a unique role in the folded structure and the binding site of many proteins, and its fluorescence properties make it very useful in studying the structures and dynamics of protein molecules in solution. An analysis has been made of its environment and the geometry of its interaction with neighbors using 719 Trp residues in 180 different protein structures. The distribution of the number of partners interacting with the Trp aromatic ring shows a peak at 6 (considering protein residues only) and 8 (including water and substrate molecules also). The means of the solvent-accessible surface areas of the ring show an exponential decrease with the increase in the number of partners; this relationship can be used to assess the efficiency of packing of residues around Trp. Various residues exhibit different propensities of binding the Trp side chain. The aromatic residues, Met and Pro have high values, whereas the smaller and polar-chain residues have weaker propensities. Most of the interactions are with residues far away in sequence, indicating the importance of Trp in stabilizing the tertiary structure. Of all the ring atoms NE1 shows the highest number of interactions, both along the edge (hydrogen bonding) as well as along the face. Various weak but specific interactions, engendering stability to the protein structure, have been identified.